Mingyuan Ge

Porous Doped Silicon Nanowires for Lithium Ion Battery Anode with Long Cycle Life

Porous silicon nanowires have been well studied for various applications; however, there are only very limited reports on porous silicon nanowires used for energy storage. Here, we report both experimental and theoretical studies of porous doped silicon nanowires synthesized by direct etching of boron-doped silicon wafers. When using alginate as a binder, porous silicon nanowires exhibited superior electrochemical performance and long cycle life as anode material in a lithium ion battery. Even after 250 cycles, the capacity remains stable above 2000, 1600, and 1100 mAh/g at current rates of 2, 4, and 18 A/g, respectively, demonstrating high structure stability due to the high porosity and electron conductivity of the porous silicon nanowires. A mathematic model coupling the lithium ion diffusion and the strain induced by lithium intercalation was employed to study the effect of porosity and pore size on the structure stability. Simulation shows silicon with high porosity and large pore size help to stabilize the structure during charge/discharge cycles.

Large-Scale Synthesis of SnO2 Nanosheets with High Lithium Storage Capacity

In this communication, we successfully synthesized a new SnO(2) nanoarchitecture: extremely thin sheets, with minimum thicknesses of 1.5-3.0 nm. The products were prepared through a facile hydrothermal treatment using tin dichloride as the precursor. Planar or scrolled SnO(2) sheets were carefully examined by transmission electron microscopy. The assemblies of these sheets have a high BET surface area of 180.3 m(2)/g and extraordinarily large pore volume of 1.028 cm(3)/g. They also exhibit a high lithium storage capacity and excellent cyclability due to its nanometer-sized frame and breathable characteristic.

Scalable preparation of porous silicon nanoparticles and their application for lithium-ion battery anodes

Nanostructured silicon has generated significant excitement for use as the anode material for lithium-ion batteries; however, more effort is needed to produce nanostructured silicon in a scalable fashion and with good performance. Here, we present a direct preparation of porous silicon nanoparticles as a new kind of nanostructured silicon using a novel two-step approach combining controlled boron doping and facile electroless etching. The porous silicon nanoparticles have been successfully used as high performance lithium-ion battery anodes, with capacities around 1,400 mA·h/g achieved at a current rate of 1 A/g, and 1,000 mA·h/g achieved at 2 A/g, and stable operation when combined with reduced graphene oxide and tested over up to 200 cycles. We attribute the overall good performance to the combination of porous silicon that can accommodate large volume change during cycling and provide large surface area accessible to electrolyte, and reduced graphene oxide that can serve as an elastic and electrically conductive matrix for the porous silicon nanoparticles.Graphical abstract

Review of porous silicon preparation and its application for lithium-ion battery anodes

Silicon is of great interest for use as the anode material in lithium-ion batteries due to its high capacity. However, certain properties of silicon, such as a large volume expansion during the lithiation process and the low diffusion rate of lithium in silicon, result in fast capacity degradation in limited charge/discharge cycles, especially at high current rate. Therefore, the use of silicon in real battery applications is limited. The idea of using porous silicon, to a large extent, addresses the above-mentioned issues simultaneously. In this review, we discuss the merits of using porous silicon for anodes through both theoretical and experimental study. Recent progress in the preparation of porous silicon through the template-assisted approach and the non-template approach have been highlighted. The battery performance in terms of capacity and cyclability of each structure is evaluated.

Solution ionic strength engineering as a generic strategy to coat graphene oxide (GO) on various functional particles and its application in high-performance lithium-sulfur (Li-S) batteries.

A generic and facile method of coating graphene oxide (GO) on particles is reported, with sulfur/GO core-shell particles demonstrated as an example for lithium-sulfur (Li-S) battery application with superior performance. Particles of different diameters (ranging from 100 nm to 10 μm), geometries, and compositions (sulfur, silicon, and carbon) are successfully wrapped up by GO, by engineering the ionic strength in solutions. Importantly, our method does not involve any chemical reaction between GO and the wrapped particles, and therefore, it can be extended to vast kinds of functional particles. The applications of sulfur/GO core-shell particles as Li-S battery cathode materials are further investigated, and the results show that sulfur/GO exhibit significant improvements over bare sulfur particles without coating. Galvanic charge-discharge test using GO/sulfur particles shows a specific capacity of 800 mAh/g is retained after 1000 cycles at 1 A/g current rate if only the mass of sulfur is taken into calculation, and 400 mAh/g if the total mass of sulfur/GO is considered. Most importantly, the capacity decay over 1000 cycles is less than 0.02% per cycle. The coating method developed in this study is facile, robust, and versatile and is expected to have wide range of applications in improving the properties of particle materials.

Black Arsenic–Phosphorus: Layered Anisotropic Infrared Semiconductors with Highly Tunable Compositions and Properties

New layered anisotropic infrared semiconductors, black arsenic-phosphorus (b-AsP), with highly tunable chemical compositions and electronic and optical properties are introduced. Transport and infrared absorption studies demonstrate the semiconducting nature of b-AsP with tunable bandgaps, ranging from 0.3 to 0.15 eV. These bandgaps fall into the long-wavelength infrared regime and cannot be readily reached by other layered materials.

On the origin of ferromagnetism in CeO2 nanocubes

Magnetic behaviors of pure CeO2 nanoparticles and nanocubes have been investigated both experimentally and theoretically. It is found that monodisperse CeO2 nanocubes with an average size of 5.3 nm do show ferromagnetic behavior at ambient temperature with a saturation magnetization of about 5.7 memu/g and coercive force of about 69 Oe. First-principles calculations reveal that oxygen vacancies in pure CeO2 cause spin polarization of f electrons for Ce ions surrounding oxygen vacancies, resulting in net magnetic moment for pure CeO2 samples. One oxygen vacancy at surface can induce more magnetic moments than those induced by one oxygen vacancy in bulk. The results obtained here provide evidence that pure CeO2 sample with oxygen vacancies can indeed have magnetic behavior. This study will stimulate more investigations for understanding the origin of ferromagnetic TM-doped CeO2 (TM=3d transition metals) in a particular case and TM-doped semiconductor oxides in general at ambient temperature.

Graphene-oxide-coated LiNi0.5Mn1.5O4 as high voltage cathode for lithium ion batteries with high energy density and long cycle life

Lithium ion batteries are receiving enormous attention as power sources and energy storage devices in the renewable energy field. With the ever increasing demand for higher energy and power density, high voltage cathodes have emerged as an important option for new generation batteries. Here, we report graphene-oxide-coated LiNi0.5Mn1.5O4 as a high voltage cathode and demonstrate that the batteries showed superior cycling performance for up to 1000 cycles. Mildly oxidized graphene oxide coating was found to improve the battery performance by enhancing the conductivity and protecting the cathode surface from undesired reactions with the electrolyte. As a result, the graphene-oxide-coated high voltage cathode LiNi0.5Mn1.5O4 showed 61% capacity retention after 1000 cycles in the cycling test, which converts to only 0.039% capacity decay per cycle. At large current rates of 5 C, 7 C and 10 C, the batteries were able to deliver 77%, 66% and 56% of the 1 C capacity, respectively (1 C = 140 mA g−1). In contrast, the LiNi0.5Mn1.5O4 cathode without graphene oxide coating showed 88.7% capacity retention after only 100 cycles. The promising results demonstrated the potential of developing high energy density batteries with the high voltage cathode LiNi0.5Mn1.5O4 and improving the battery performance by surface modification with mildly oxidized graphene oxide.

Stability, electronic, and magnetic behaviors of Cu adsorbed graphene: A first-principles study

Stable configurations, electronic structures, and magnetic behaviors for Cu single atom and dimer adsorption on graphene have been investigated by first-principles calculations, using both gradient generalized approximation (GGA) and GGA+U methods. It is found that Cu single atom, sitting above a carbon atom, is the most stable configuration, while the most stable configuration for Cu dimer is perpendicular to the graphene plane above the bridge site. In both calculations, magnetic moments were detected in Cu single-atom-adsorbed graphene, which is mainly caused by the unsaturated s-electrons of the Cu atom. No magnetic moment was observed in Cu dimer-adsorbed graphene in both cases, but a band gap was observed in the GGA+U calculation, while a metallic system was observed in the GGA calculation. This demonstrates that the electronic structure of graphene can be modified via Cu adsorption. The Cu dimer, sitting parallel to the graphene plane above the C atoms, offers the possibility for the formation of C...

Reversible Semiconducting-to-Metallic Phase Transition in Chemical Vapor Deposition Grown Monolayer WSe2 and Applications for Devices

Two-dimensional (2D) semiconducting monolayer transition metal dichalcogenides (TMDCs) have stimulated lots of interest because they are direct bandgap materials that have reasonably good mobility values. However, contact between most metals and semiconducting TMDCs like 2H phase WSe2 are highly resistive, thus degrading the performance of field effect transistors (FETs) fabricated with WSe2 as active channel materials. Recently, a phase engineering concept of 2D MoS2 materials was developed, with improved device performance. Here, we applied this method to chemical vapor deposition (CVD) grown monolayer 2H-WSe2 and demonstrated semiconducting-to-metallic phase transition in atomically thin WSe2. We have also shown that metallic phase WSe2 can be converted back to semiconducting phase, demonstrating the reversibility of this phase transition. In addition, we fabricated FETs based on these CVD-grown WSe2 flakes with phase-engineered metallic 1T-WSe2 as contact regions and intact semiconducting 2H-WSe2 as active channel materials. The device performance is substantially improved with metallic phase source/drain electrodes, showing on/off current ratios of 10(7) and mobilities up to 66 cm(2)/V·s for monolayer WSe2. These results further suggest that phase engineering can be a generic strategy to improve device performance for many kinds of 2D TMDC materials.